64 ORIGIN OF SPECIES 



by strongly marked and important characters, "there is no 

 possible test but individual opinion to determine which of 

 them shall be considered as species and which as varieties." 

 Lastly, representative species fill the same place in the nat' 

 ural economy of each island as do the local forms and sub- 

 species; but as they are distinguished from each other by a 

 greater amount of difference than that between the local 

 forms and sub-species, they are almost universally ranked 

 by naturalists as true species. Nevertheless, no certain cri- 

 terion can possibly be given by which variable forms, local 

 forms, sub-species, and representative species can be 

 recognised. 



Many years ago, when comparing, and seeing others com- 

 pare, the birds from the closely neighbouring islands of the 

 Galapagos archipelago, one with another, and with those 

 from the American mainland, I was much struck how entirely 

 vague and arbitrary is the distinction between species and 

 varieties. On the islets of the little Madeira group there are 

 many insects which are characterised as varieties in Mr. 

 Wollaston's admirable work, but which would certainly 

 be ranked as distinct species by many entomologists. Even 

 Ireland has a few animals, now generally regarded as 

 varieties, but which have been ranked as species by some 

 zoologists. Several experienced ornithologists consider our 

 British red grouse as only a strongly-marked race of Nor- 

 wegian species, whereas the greater number rank it as an 

 undoubted species peculiar to Great Britain. A wide dis- 

 tance between the homes of two doubtful forms leads many 

 naturalists to rank them as distinct species ; but what dis- 

 tance, it has been well asked, will suffice; if that between 

 America and Europe is ample, will that between Europe and 

 the Azores, or Madeira, or the Canaries, or between the sev- 

 eral islets of these small archipelagos, be sufficient? 



Mr. B. D. Walsh, a distinguished entomologist of the 

 United States, has described what he calls Phytophagic 

 varieties and Phytophagic species. Most vegetable-feeding 

 insects live on one kind of plant or on one group of plants; 

 some feed indiscriminately on many kinds, but do not in 

 consequence vary. In several cases, however, insects found 

 living on different plants, have been observed by Mr. Walsh 



